Aircraft Fuel Systems — Page 316, Lesson 392

We're starting the Aircraft Fuel Systems chapter, and I want to begin with the fuels themselves, because everything downstream—tank design, pumps, engine feed—depends on what we're burning. Gas turbine engines, which power essentially all modern civilian airliners, run on kerosene-based fuels. There are two main families you need to know: AVTUR and AVTAG. AVTUR stands for Aviation Turbine Fuel. The primary type is JET A1. It's a kerosene fuel with a nominal specific gravity of 0.8 at 15°C. Specific gravity is just the density of the fuel compared to water—so JET A1 is 80% as dense as water. It has a flash point of 38°C, which is the lowest temperature at which the fuel gives off enough vapour to ignite in the presence of a flame. And it has a waxing point of -47°C, the temperature at which wax crystals start to form and could clog filters. The second AVTUR type is JET A. It's very similar—same specific gravity, same flash point of 38°C—but its waxing point is higher, at -40°C. That means it starts to wax at a warmer temperature than JET A1. JET A is normally only available in the USA. Now AVTAG, which stands for Aviation Turbine Gasoline. Its type is JET B. This is a wide-cut fuel—a mix of gasoline and kerosene. It has a nominal specific gravity of 0.77 at 15°C, slightly lighter than JET A1. Its flash point is much lower, as low as -20°C, and it has a wider boiling range than JET A1. Its waxing point is -60°C, so it resists waxing at much colder temperatures. JET B can be used as an alternative to JET A1, but here's the catch: it has a wider range of flammability. That means it's easier to ignite across a broader range of conditions, which is why it's not generally used in civilian aircraft. It's more of a military fuel. Now, a couple of practical points about identifying fuel. Turbine fuels are not dyed for identification. They retain their natural colour, which can range from a straw yellow to completely colourless. So you can't tell the grade by looking at it—you rely on documentation and testing. But you can learn something from a cloudy sample. If you take a fuel sample and it looks cloudy or hazy, the behaviour of that cloud tells you what's in it. If the cloudiness rises quite rapidly towards the top of the sample, air is present. If the cloud falls quite slowly towards the bottom, water is present. A cloudy appearance usually indicates the presence of water. Finally, let's talk about additives. A number of additives may be blended into the fuel, either at the refinery or at the airfield, to improve its operating ability. The most popular is FSII—Fuel System Icing Inhibitor. Here's the problem FSII solves. A certain amount of water is present in all fuel. As an aircraft climbs to altitude, the fuel is cooled, and the amount of dissolved water it can hold is reduced. Water droplets form, and as temperature drops further, they turn to ice crystals which can block fuel system components. That's the icing problem. There's also a biological problem. A microbiological fungus called Cladosporium Resinae is present in all turbine fuels. It grows rapidly in the presence of water, forming long green filaments that can block fuel system components. And its waste products are corrosive, especially to fuel tank sealing substances. FSII contains an icing inhibitor and a fungal suppressant to combat both of these problems. So it's doing double duty—keeping ice from forming and keeping fungus from growing. Let me show you the layout of a simple gravity feed system so you can see where all this fuel ends up. That's a single-engine light aircraft gravity feed system. The tank sits above the engine, and fuel flows down by gravity—no pump needed. The principle relies on the height of the fuel column, which is exactly what that earlier figure about fluid pressure in an open container was showing you. For an open container, the pressure exerted by the fluid depends only on the height of the fluid column. That's the whole basis of gravity feed—the taller the fuel column above the carburettor, the greater the pressure pushing fuel through the system. So to summarise what we've covered: AVTUR covers JET A1 and JET A, both kerosene fuels. AVTAG covers JET B, the wide-cut gasoline/kerosene mix. Turbine fuels aren't dyed. Cloudy fuel behaviour tells you whether it's air or water. And FSII is the additive that fights icing and fungal growth from Cladosporium Resinae.